In recent years, an LCD device of a TFT system has widely been used as a display device. The LCD device has a glass substrate on which the TFT (thin film transistor) is formed in a matrix shape, this TFT driving the liquid crystal in a vertical direction of the TFT. The TFT is formed with an insulating film or a polysilicon film laminated on the glass substrate. As the glass substrate, a soda glass cheaper than a quartz glass has been used. The soda glass has a low softening point of about 500° C. as compared with the quartz glass. In addition, Na included in the soda glass is diffused at a high temperature environment. Thus, a film forming technique at 400° C. or lower has been demanded. In addition, a film quality of a film thus generated is demanded having a high quality closely to that of the film formed at a higher temperature. In addition, in recent years, as represented by a flexible information terminal (flexible PC, cellular phone), a silicon device preparing technique on an organic (flexible) substrate such as plastics (polyimide) or so forth becomes important. In this case, the process temperature is 250° C. or lower, as viewed from the heat-resistant temperature of polyimide and so forth.
As these insulating films, silicon oxide films are mainly used. A film forming method mainly includes a technique in which an SiO2 film is deposited on a polysilicon (for example, a film thickness is 50 nm) prepared on a glass (or an organic material) through a thermal CVD or a plasma CVD. In a case of the thermal CVD, SiH4+O2 is mainly used. It is, however, noted that many problems such that the insulating film prepared at 300° C. or lower contains a large quantity of impurities or moisture, has a low compactness, has many pinholes and particles, and has a low insulating withstanding voltage and many leak currents are raised.
Then, to solve these problems, the plasma CVD method is used. For example, according to non-patent literature 1 (J. Vac. Sci. Technol. A21, 728 (2003)), the preparation of the insulating film having a good quality of the insulating withstanding voltage of 4 MV/cm, an interface level density of 1012 [eV−1 cm−2], and a stationary electric charge density of 1011 [cm−2] at a substrate temperature of 100° C. has succeeded through the CVD using SiH4+O2 gas within an ECR (Electron Cyclotron Resonance) plasma reaction furnace.
However, if a plasma CVD method is used, the following problem occurs. Due to a plasma reaction in a gaseous phase, many particles are found, a step coverage is low, a process window (a film forming temperature, a gas pressure, and an opposing inter-electrode distance, and so forth) to realize an ideal film density (2.2 g/cm3) and a composition (Si:O=1:2) is narrow. Due to the presence of many particles, it is necessary to clarify a film forming chamber frequently.
To relieve the problem described hereinabove, as the film forming method which can use a high safety reaction gas, can set easily the film forming condition, and can suppress the development of the particles easily, a thermal CVD process under a mixture gas including an organic silicon material (easier in handling than SiH4) having Si—O bond and Si—H bond and ozone has been proposed. A high oxidizing ability of ozone gas to the organic material is utilized.
According to patent literature 1 (Japanese Publication Patent Application (tokkai) No. Heisei 8-31815), the deposition of SiO2 film having a favorable step coverage at 400° C. due to the thermal CVD under an atmospheric pressure using TMS (Trimethoxysilane), TES (Triethoxysilane), TEOS (Tetraethylorthosilicate), and the ozone gas has succeeded. It is noted that, ozone is a gas having the thermal decomposition and a concentration of the ozone gas gives a large effect on the film quality. Hence, by using nitrogen as a carrier gas and diluting ozone gas to a sufficiently low concentration at which the ozone gas can stably be present, an effective supply of ozone gas, a uniform development of CVD reaction at the proximity of the substrate surface and a uniform film deposition are realized.
On the other hand, according to patent literature 2 (Japanese Publication Patent Application (tokkai) No. Heisei 5-259155), it is known that, as the concentration of the ozone gas used becomes higher, the concentration of impurities such as hydrocarbon present in the film becomes lower so that a moisture absorption resistant characteristic becomes favorable, an insulating characteristic becomes favorable, and a leak current becomes small. That is to say, in a case where the ozone gas is used, there is a trade-off relationship between a uniformity in CVD film (film thickness and film quality) and a high quality of the film. Although utilizing a high organic material utilizing a high oxidizing ability of the ozone gas to organic material, a process in which reactivity is controlled using the high concentration ozone gas is not yet architected.
Furthermore, at a low temperature process equal to or lower than 300° C., even in the CVD using an ozone containing gas, a contamination of C-series organic material and water (and hydrogen or OH) into the film due to byproducts formed by the decomposition at the gaseous phase of the organic material gas cannot perfectly be eliminated. For example, as reported in non-patent literature 2 (J. Vac. Sci. Technol. B8, 533 (1990)), As-deposited film is porous as suggested from the fact that the film after the deposition in a buffered hydrofluoric acid solution indicates an etching speed faster than a thermal oxide film by ten times or more. If RTA (Rapid Thermal Annealing) after the deposition is performed, a high density of the film can be achieved (appearing as a reduction of the etching speed of film). However, it is necessary to heat the film at about 1000° C. to obtain a high density film approximate to the thermal oxide film. This high temperature heating process cannot be used in the film forming process on the glass or organic substrate as described above.
Non-patent literature 1: J. Vac. Sci. Technol. A21, 728 (2003)
Patent literature 1: Japanese Publication Patent Application No. Heisei 8-31815
Patent literature 2: Japanese Publication Patent Application No. Heisei 5-259155
Non-patent literature 2: J. Vac. Sci. Technol. B8, 533 (1990)